Numerical modelling of free field traffic-induced vibrations

This paper deals with the numerical modelling of free field traffic-induced vibrations during the passage of a vehicle on an uneven road. The road unevenness subjects the vehicle to vertical oscillations that cause dynamic axle loads. The latter are calculated from the vehicle transfer functions and the frequency content of the road profile as experienced by the vehicle axles. A transfer function between the source and the receiver that accounts for the dynamic interaction between the road and the soil is used to calculate the free field response. Its calculation is based on a dynamic substructure method, using a boundary element method for the soil and an analytical beam model for the road. The methodology is validated with analytical results and is finally illustrated by a numerical example where the free field vibrations during the passage of a vehicle on a traffic plateau are considered.     

A 2.5D coupled FE-BE model for the prediction of railway induced vibrations

Ground vibrations induced by railway traffic at grade and in tunnels are often studied by means of two-and-half dimensional (2.5D) models that are based on a Fourier transform of the coordinate in the longitudinal direction of the track. In this paper, the need for 2.5D coupled finite element-boundary element models is demonstrated in two cases where the prediction of railway induced vibrations is considered. A recently proposed novel 2.5D methodology is used where the finite element method is combined with a boundary element method, based on a regularized boundary integral equation. In the formulation of the boundary integral equation, Green's functions of a layered elastic halfspace are used, so that no discretization of the free surface or the layer interfaces is required. In the first case, two alternative models for a ballasted track on an embankment are compared. In the first model, the ballast and the embankment are modelled as a continuum using 2.5D solid elements, whereas a simplified beam representation is adopted in the second model. The free field vibrations predicted by both models are compared to those measured during a passage of the TGVA at a site in Reugny (France). A very large difference is found for the free field response of both models that is due to the fact that the deformation of the cross section of the embankment is disregarded in the simplified representation. In the second case, the track and free field response due to a harmonic load in a tunnel embedded in a layered halfspace are considered. A simplified methodology based on the use of the full space Green's function in the tunnel–soil interaction problem is investigated. It is shown that the rigorous finite element-boundary element method is required when the distance between the tunnel and the free surface and the layer interfaces of the halfspace is small compared to the wavelength in the soil.     

Numerical evaluation of the dynamic response of pipelines to vibrations induced by the operation of a pavement breaker

This paper studies the response of pipelines to vibrations induced by the operation of a pavement breaker during the rehabilitation of concrete pavements. An efficient two-and-a-half-dimensional (2.5D) formulation is employed, where the geometry of the structure and the soil is assumed to be invariant in the longitudinal direction, allowing for a Fourier transform of the longitudinal coordinate y along the structure to the wavenumber k_y. The dynamic soil–structure interaction problem is solved by means of a 2.5D coupled finite element–boundary element (FE–BE) method using a subdomain formulation. The numerical model is verified by means of results available in the literature for a buried pipeline subjected to incident P- and SV-waves with an arbitrary angle of incidence. The presented methodology is capable to incorporate any type of incident wave field induced by earthquakes, construction activities, traffic, explosions or industrial activities. The risk of damage to a high pressure steel natural gas pipeline and a concrete sewer pipe due to the operation of a pavement breaker is assessed by means of the 2.5D coupled FE–BE methodology. It is observed that the stresses in the steel pipeline due to the operation of the pavement breaker are much lower than those induced by the operating internal pressure. The steel pipeline behaves in the linear elastic range under the combined effect of the loadings, indicating that damage to steel pipelines close to the road due to the operation of a pavement breaker is unlikely. The maximum principal stress in the concrete pipe, on the other hand, remains only slightly lower than the specified tensile strength. The decision to use a pavement breaker should hence be taken with care, as its operation may induce tensile stresses in concrete sewer pipes which are of the same order of magnitude as the tensile strength of the concrete. Assessing the risk of damage by means of vibration guidelines based on the peak particle velocity (PPV) gives, for the particular cases considered, qualitatively similar results.     

Local amplification of deep mining induced vibrations part.2: Simulation of ground motion in a coal basin

This work investigates the impact of deep coal mining induced vibrations on surface constructions using numerical tools. An experimental study of the geological site amplification and of its influence on mining induced vibrations has already been published in the previous paper (Part 1: Experimental evidence for site effects in a coal basin). Measurements have shown the existence of an amplification area in the southern part of the basin where drilling data have shown the presence of particularly fractured and soft stratigraphic units. The present study, using the boundary element method (BEM) in the frequency domain, first investigates canonical geological structures in order to get general results for various sites. The amplification level at the surface is given as a function of the shape of the basin and of the velocity contrast with the bedrock. Next, the particular coal basin previously studied experimentally (Driad-Lebeau et al. [1]) is modeled numerically by BEM. The amplification phenomena characterized numerically for the induced vibrations are found to be compatible with the experimental findings such as: amplification level, frequency range and location. Finally, the whole work was necessary to fully assess the propagation and amplification of mine induced vibrations. The numerical results quantifying amplification can also be used to study other coal basins or various types of alluvial sites.     

水平成层场地地震反应的集中质量切比雪夫谱元分析方法

提出了一种用于水平成层场地地震反应分析的时域高阶显式算法. 首先,将覆盖土层和基岩划分为若干个切比雪夫谱单元,在模型底部设置多次透射人工边界;其次,以切比雪夫正交多项式构建高阶单元位移模式,通过高斯?洛巴托积分严格导出对角形式的切比雪夫谱单元集中质量矩阵,结合中心差分时域逐步积分格式,建立了高效的集中质量切比雪夫谱元波动模拟方法;最后,利用日本Kik-net强震台网提供的不同类型场地上获得的实际地震观测记录检验了本文方法的有效性. 该方法避免了传统切比雪夫谱元法由于具有一致质量矩阵形式而造成的计算效率不高的问题。数值结果表明,本文方法能够较好地预测Ⅰ₁,Ⅱ和Ⅳ类场地在较弱地震和中等强度地震作用下的地面运动特征,每个波长内仅需布置少量单元即可取得较高精度的计算结果。      

Experimental and numerical analyses of vibrations induced by high-speed trains on the Córdoba–Málaga line

This paper has two main purposes. One is to present and analyse soil and structural vibration data obtained experimentally during certification testing of the high-speed train line between Córdoba and Málaga (Spain) that was opened on December 2007. The second is to show the capabilities of a three-dimensional boundary element method (BEM)/finite element method (FEM) numerical approach for the analysis of train induced vibrations. The model can represent local soil conditions, discontinuities such as underpasses, as well as structures placed next to the rail track. Vibrations in those structures can be computed taking into account, in a rigorous way, dynamic soil–structure interaction and local soil properties. Experimental and numerical results at several points near the track are compared. Results for an overhead contact support structure are also evaluated. The comparison of numerically predicted and recorded results shows that the model is reliable for predicting the amplitude of vibrations produced in the soil and nearby structures by high-speed trains.     

Symmetric modes of vibration for A circular plate using finite dynamic elements

The dynamic element method has been shown previously to provide a computational advantage over the ordinary finite element method for various beam elements. The Taylor expansions are computed here for the dynamic shape functions (two terms) and dynamic stiffness matrix (four terms) for the axisymmetric vibrations of an annular plate element. The complicated matrices which result are made more tractable by expressing them as power series in powers of the aspect ratio. The percentage error in the natural frequencies is then calculated using both the two- and the three-term dynamic stiffness matrix, demonstrating the increased accuracy for a given number of elements.     

Boundary integral equation method for the diffraction of elastic waves using simplified green's functions

An alternate formulation of the ‘substructure deletion method’ suggested by Dasgupta in 1979¹ has been successfully implemented. The idea is to utilize simple Green's functions developed for a surface problem to replace the more complicated Green's functions required for embedded problems while still being able to generate an accurate solution. Since the exterior medium is usually represented by a continuum model, the interior medium in the present approach will also be represented by a continuum model rather than a finite element model as suggested originally, thereby eliminating the incompatibility between the solutions of the interior and exterior media. Detailed studies of the method's accuracy and limitations were performed using two-dimensional examples in wave scattering of canyons and alluvial valleys, problems which are more suitable for this method than the embedded foundation problem. The results obtained indicate that the alternate formulation gives accurate results only when the vertical dimension of the scattering object is not too large; if the aspect ratio (vertical over lateral) exceeds a certain limit, the results will not approach the known results given by boundary integral equation solutions or indirect boundary integral equations no matter what the refinement of the model may be. The greatest advantage of the present method is that the task of calculating Green's functions is reduced significantly; computational time using this new formulation is approximately five times less than for conventional boundary integral equation methods.     

Transient elastodynamic analysis of three-dimensional problems by boundary element method

An advanced implementation of the direct boundary element method applicable to transient problems involving three-dimensional solids of arbitrary shape and connectivity is presented. The work first focuses on the formulation of the method, followed by a discussion of the fundamental singular solutions. Subsequently, a family of isoparametric boundary elements is introduced, along with the necessary numerical integration techniques as well as the solution algorithm. Numerical examples are presented, which demonstrate the unconditional stability and high accuracy of this dynamic analysis technique.     

三维直流电阻率有限元-无限元耦合数值模拟

为解决传统有限元截断边界所引起的问题,本文提出了一种新的三维直流电阻率有限元-无限元耦合数值模拟方法.首先推导了无限元三维单元映射函数,然后提出了一种全新的最优的无限元形函数并与多种其他形函数进行了对比,随后将其与非结构化四面体有限元相结合,取代了传统的混合边界条件,使得电位在无限域内连续并在无限远处衰减为零,最终形成的左端矩阵稀疏对称并与场源位置无关.数值计算表明,该方法可以在近似测区大小的计算范围内得到与混合边界条件相当的计算精度,优于相同计算范围下齐次边界条件的解,有利于减少计算节点数;由于左端矩阵不随场源位置改变,有利于加速反演计算.     

Experimental validation of a numerical prediction model for free field traffic induced vibrations by in situ experiments

This paper deals with the validation of a numerical model for traffic induced vibrations. Road unevenness subjects the vehicle to vertical oscillations that cause dynamic axle loads, which generate waves propagating in the subsoil. A 2D vehicle model is used for the calculation of the axle loads from the longitudinal road profile. The free field soil response is calculated with the dynamic Betti–Rayleigh reciprocity theorem, using a transfer function between the road and the receiver that accounts for dynamic road–soil interaction. The validation relies on the measured response of the vehicle's axles and the soil during the passage of a truck on an artificial unevenness with speeds varying from 30 to 70 km/h. The agreement between the numerical and the experimental results is good: the influence of the vehicle speed and the distance from the road is well predicted, while the ratio of the predicted and the measured PPV is less than two.     

Scattering of seismic waves by cracks in multi-layered geological regions: I. Mechanical model

In this work, a hybrid boundary integral equation method (BIEM) is developed, based on both displacement and hypersingular traction formulations, for the analysis of time-harmonic seismic waves propagating through cracked, multi-layered geological regions with surface topography and under plane strain conditions. Specifically, the displacement-based BIEM is used for a multi-layered deposit with interface cracks, while the regularized, traction-based BIEM is used when internal cracks are present within the layers. The standard uni-dimensional boundary element with parabolic shape functions is employed for discretizing the free surface and the layer interfaces, while special discontinuous boundary elements are placed near the crack tips to model the asymptotic behaviour of both displacements and tractions. This formulation yields displacement amplitudes and phase angles on the free surface of a geological deposit, as well as stress intensity factors near the tips of the cracks. Finally, in the companion paper, numerical results are presented which show that both scattered wave and stress concentration fields are sensitive to the incidence seismic wave parameters and to specific site conditions such as surface topography, layering, the presence of cracks and crack interaction.     

The effect of road unevenness on the dynamic vehicle response and ground-borne vibrations due to road traffic

In this paper, the relation between road unevenness, the dynamic vehicle response, and ground-borne vibrations is studied. In situ measurements of road unevenness and the dynamic vehicle response for six roads with different types of pavement are supplemented by numerical predictions of ground vibrations. The predictions are performed in two stages. In the first stage, the dynamic vehicle response is computed based on the measured road unevenness. The vehicle model is validated by comparing the predicted and measured vehicle response and subsequently used to predict the dynamic vehicle loads. In the second stage, the dynamic road–soil interaction problem is considered and the transfer functions between the road and the soil are computed. The effect of the pavement type (continuous, jointed, or composed of individual pavers) on the road–soil transfer functions is investigated and the free field vibrations are calculated using the dynamic vehicle loads computed in the first stage. The predicted free field vibrations are validated by measurements at one of the measurement sites before and after rehabilitation of a deteriorated concrete pavement. Finally, the results are used to investigate the relation between indicators of road unevenness such as the ISO 8608 road class, the International Roughness Index, and the coefficient of evenness on one hand, and the dynamic vehicle response and level of ground-borne vibration on the other hand.     

Attenuation of ground vibrations due to different technical sources

The attenuation of technically induced surface waves is studied theoretically and experimentally.In this paper, nineteen measurements of ground vibrations induced by eight different technical sources including road and rail traffic, vibratory and impulsive construction work or pile driving, explosions, hammer impulses and mass drops are described, and it is shown that the technically induced ground vibrations exhibit a power-law attenuation v～r-q where the exponents q are in the range of 0.5 to 2.0 and depend on the source types.Comparisons performed demonstrate that the measured exponents are considerably higher than theoretically expected.Some potential effects on ground vibration attenuation are theoretically analyzed.The most important effect is due to the material or scattering damping.Each frequency component is attenuated exponentially as exp(-kr), but for a broad-band excitation, the sum of the exponential laws also yields a power law but with a high exponent.Additional effects are discussed, for example the dispersion of the Rayleigh wave due to soil layering, which yields an additional exponent of 0.5 in cases of impulsive loading.     

Boundary element reservoir model for seismic analysis of gravity dams

The boundary element method has been successfully applied in the past to the analysis of hydrodynamic forces in two- and three-dimensional finite water reservoirs subjected to seismic ground motions. In extending the method to an infinite reservoir, the loss of energy due to pressure waves moving away towards infinity must be taken into account. In addition, for both finite and infinite reservoirs, energy is lost owing to partial absorption of the waves incident on a flexible bottom consisting of alluvial deposits. This paper presents the results of more recent research on the application of the boundary element method to the analysis of 2D reservoir vibration. Two different formulations are used: a constant boundary element formulation and a linear boundary element formulation. Special boundary conditions to treat infinite radiation and foundation damping have been incorporated in both formulations. Numerical results have been obtained for each of the two alternative formulations and compared against each other as well as with classical solutions and results obtained by other researchers.     

A simple boundary element formulation and its application to wavefield excited soil-structure interaction

A simple boundary element formulation which is based directly on the point load solutions for an elastic full-space is presented. It is integrated in a finite element program to calculate dynamic soil-structure interaction problems. The combined boundary and finite element method is applied to structures which are excited by horizontally propagating waves in the soil. For three different types of flexible structure-elastic beams, low and high (square) shear walls-and the corresponding rigid structures the vibration modes and the soil-structure transfer functions have been investigated. The flexible foundations display the same wave pattern as the exciting free-field of the soil, but the amplitudes are reduced with increasing frequency, depending on the stiffness or wave resistance of the structure. Rigid structures show, in part, quite different behaviour, giving free-field reductions caused by kinematic and inertial soil-structure interaction.     

Soil–structure interaction in the time domain using halfspace Green's functions

A time-domain formulation is proposed for the transient response analysis of general, three-dimensional structures resting on a homogeneous, elastic halfspace subjected to either external loads or seismic motions. The formulation consists of two parts: (a) the time domain formulation of the soil behaviour and (b) the coupling of the corresponding soil algorithms to the Finite Element Code ANSYS. As far as the structure is concerned, this coupling opens the way for the analysis of non-linear soil–structure interaction. The approach is based on halfspace Green's functions for displacements elicited by Heaviside time-dependent surface point loads. Hence, the spatial discretisation can be confined to the contact area between the foundation and the soil, i.e. no auxiliary grid beyond the foundation as for conventional boundary element formulations is required. The method is applied to analyse the dynamic response of a railway track due to a moving wheel set by demonstrating the influence of ‘through-the-soil coupling’.     

Transient analysis of wave propagation in non‐homogeneous elastic unbounded domains by using the scaled boundary finite‐element method

The scaled boundary finite‐element method has been developed for the dynamic analysis of unbounded domains. In this method only the boundary is discretized resulting in a reduction of the spatial dimension by one. Like the finite‐element method no fundamental solution is required. This paper extends the scaled boundary finite‐element method to simulate the transient response of non‐homogeneous unbounded domains with the elasticity modulus and mass density varying as power functions of spatial coordinates. To reduce the number of degrees of freedom and the computational cost, the technique of reduced set of base functions is applied. The scaled boundary finite‐element equation for an unbounded domain is reformulated in generalized coordinates. The resulting acceleration unit‐impulse response matrix is obtained and assembled with the equation of motion of standard finite elements. Numerical examples of non‐homogeneous isotropic and transversely isotropic unbounded domains demonstrate the accuracy of the scaled boundary finite‐element method. Copyright © 2006 John Wiley & Sons, Ltd.     

A lumped mass Chebyshev spectral element method and its application to structural dynamic problems

A diagonal or lumped mass matrix is of great value for time-domain analysis of structural dynamic and wave propagation problems, as the computational efforts can be greatly reduced in the process of mass matrix inversion. In this study, the nodal quadrature method is employed to construct a lumped mass matrix for the Chebyshev spectral element method (CSEM). A Gauss-Lobatto type quadrature, based on Gauss-Lobatto-Chebyshev points with a weighting function of unity, is thus derived. With the aid of this quadrature, the CSEM can take advantage of explicit time-marching schemes and provide an efficient new tool for solving structural dynamic problems. Several types of lumped mass Chebyshev spectral elements are designed, including rod, beam and plate elements. The performance of the developed method is examined via some numerical examples of natural vibration and elastic wave propagation, accompanied by their comparison to that of traditional consistent-mass CSEM or the classical finite element method (FEM). Numerical results indicate that the proposed method displays comparable accuracy as its consistent-mass counterpart, and is more accurate than classical FEM. For the simulation of elastic wave propagation in structures induced by high-frequency loading, this method achieves satisfactory performance in accuracy and efficiency.

     

An absorbing boundary condition for wave propagation in saturated poroelastic media — Part II: Finite element formulation

In a finite element formulation for dynamic soil-structure interaction, an absorbing boundary condition is needed to model wave propagation towards infinity. When the soil is saturated, its dynamic behaviour can be modelled by means of Biot's poroelastic theory. In Part I (Degrande, G. & De Roeck, G., Soil Dynamics & Earthquake Eng., 1993, 12(7), 411-21), a local absorbing boundary condition for wave propagation in saturated poroelastic media has been developed. In the present paper, this boundary condition is implemented in an irreducible finite element formulation for a compressible pore fluid. Spurious reflections for oblique incident waves on the absorbing boundary contribute to the solution errors. Therefore, a spectral element method, based on classical analytical solution techniques, is used to assess the accuracy of the finite element formulation.